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    <title>Document</title>
    <script>
        // 用户输入年份，输出当前年份2月份的天数
        function backDay() {
            let year = prompt('请输入年份');
            let flag = isLeapYear(year);
            if (flag) {
                console.log(`${year} 年的2月份有29天`);
            } else {
                console.log(`${year} 年是平年,2月份有28天`);
            }
        }

        // 判断闰年函数
        function isLeapYear(year) {
            let result = false;
            if (((year % 4 == 0) && (year % 100 != 0)) || (year % 100 == 0 && year % 400 == 0)) {
                result = true;
            }
            return result;
        }

        
        backDay();
    </script>
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</html>